Organic polymer-based electroluminescent devices (OLEDs) have the potential for providing inexpensive alternatives to alphanumeric displays and x-y addressable displays. Typically, an OLED consists of a transparent substrate coated with a transparent conducting material, such as Indium Tin oxide (ITO), one to five organic layers and a cathode made by evaporation or sputtering a metal of low work function characteristics, such as Ca or Mg. The organic layers are chosen so as to provide charge injection and transport from both electrodes to the electroluminescent organic layer (EL) where charges recombine emitting light. Usually there are one or two organic hole transport layers (HTL) between the ITO and EL, as well as one or two electron injection and transporting layers (EL) between the cathode and the EL.
OLED's are fabricated usually by either vacuum depositing the organic layers (typically dyes of low molecular weight) onto the substrate, or by spin casting from solutions in the liquid state. In the following discussion, a vacuum deposited dye device will be referred to as "D-OLED", and a polymer-based device will be referred to as a "P-OLED". In some devices, a mixture of both techniques is utilized.
To define pixels and other patterns (such as icons) on a display, the ITO, organic layers, and cathode must be patterned. The patterning of the ITO layer may be carried out by conventional photolithographic techniques.
However, the patterning of the organic layers and the cathode presents difficulties that inhibit the commercial development of OLEDs. In particular, it is difficult to obtain high-resolution patterns by vapor deposition of materials through shadow masks. In order to circumvent this problem, typical D-OLED's use a "self patterning" approach in which two layers of photoresist are patterned on ITO using microlithography to create a topology that interrupts the continuity of the dye and cathode films into discrete rows (or columns) when the cathode is deposited. While this methodology provides high-resolution patterns, the construction of multi-color displays by this technique presents problems. In particular, designs in which different color pixels are constructed by depositing different light emitting dyes are difficult to construct.
Spin cast polymers allow the generation of multi-color displays in which different pixels are constructed from different polymers. Systems for photolithographically patterning the various polymer layers have been developed for some of these polymers. For example, the patterning of organic polymers for use in generating multi-color displays is taught in PCT application PCT/GB97/00995. However, the deposition of the cathode in P-OLED's is typically carried out by depositing the cathode through a shadow mask. This shadow mask process has lower resolution than the photolithography-based methods used in D-OLEDs.
A second problem with P-OLED devices is the tendency of the anode and cathode to short through the thin polymer layers. The polymer layers are typically a 0.1 .mu.m. The ITO layer over which the polymer is cast is also approximately this thick. A P-OLED device is typically constructed by etching column electrodes of ITO, covering the ITO columns with the relevant color polymers by one or more spin casting operations, and then depositing row electrodes for the cathode. The quality of the polymer layers at the points at which the layers step up and down from the ITO anodes to the spaces between the ITO anodes is often less than perfect. In particular, pin-holes or other breaks in the polymer layer are sometimes present at these points. When the cathode layer is deposited in rows over the polymer layer, shorts form between the cathode and underlying ITO anode. These shorts result in poor device yield.
Broadly, it is the object of the present invention to provide an improved method for constructing P-OLED displays.
It is a further object of the present invention to provide a method for constructing P-OLED displays that allows the resolution obtained in photolithography-based methods to be obtained in the cathode electrodes.
It is a still further object of the present invention to provide a method for constructing P-OLED displays which avoids the shorting problem between the anode and cathode electrode.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.